This application relates to semiconductor structures and processes, and particularly relates to group Ill-nitride materials systems and methods such as might be used in blue laser diodes.
The development of the blue laser light source has heralded the next generation of high density optical devices, including disc memories, DVDs, and so on. FIG. 1 shows a cross sectional illustration of a prior art semiconductor laser devices. (S. Nakamura, MRS BULLETIN, Vol. 23, No. 5, pp. 37-43, 1998.) On a sapphire substrate 5, a gallium nitride (GaN) buffer layer 10 is formed, followed by an n-type GaN layer 15, and a 0.1 xcexcm thick silicon dioxide (SiO2) layer 20 which is patterned to form 4 xcexcm wide stripe windows 25 with a periodicity of 12 xcexcm in the GaN less than 1-100 greater than direction. Thereafter, an n-type GaN layer 30, an n-type indium gallium nitride (In0.1Ga0.9N) layer 35, an n-type aluminum gallium nitride (Al0.14Ga0.86N)/GaN MD-SLS (Modulation Doped Strained-Layer Superlattices) cladding layer 40, and an n-type GaN cladding layer 45 are formed. Next, an In0.02Ga0.98N/In0.15Ga0.85N MQW (Multiple Quantum Well) active layer 50 is formed followed by a p-type Al0.2Ga0.8N cladding layer 55, a p-type GaN cladding layer 60, a p-type Al0.14Ga0.86N/GaN MD-SLS cladding layer 65, and a p-type GaN cladding layer 70. A ridge stripe structure is formed in the p-type Al0.14Ga0.86N/GaN MD-SLS cladding layer 55 to confine the optical field which propagates in the ridge waveguide structure in the lateral direction. Electrodes are formed on the p-type GaN cladding layer 70 and n-type GaN cladding layer 30 to provide current injection.
In the structure shown in FIG. 1, the n-type GaN cladding layer 45 and the p-type GaN 60 cladding layer are light-guiding layers. The n-type Al0.14Ga0.86N/GaN MD-SLS cladding layer 40 and the p-type Al0.14Ga0.86N/GaN MD-SLS cladding layer 65 act as cladding layers for confinement of the carriers and the light emitted from the active region of the InGaN MQW layer 50. The n-type In0.1Ga0.9N layer 35 serves as a buffer layer for the thick AlGaN film growth to prevent cracking.
By using the structure shown in FIG. 1, carriers are injected into the InGaN MQW active layer 50 through the electrodes, leading to emission of light in the wavelength region of 400 nm. The optical field is confined in the active layer in the lateral direction due to the ridge waveguide structure formed in the p-type Al0.14Ga0.86N/GaN MD-SLS cladding layer 65 because the effective refractive index under the ridge stripe region is larger than that outside the ridge stripe region. On the other hand, the optical field is confined in the active layer in the transverse direction by the n-type GaN cladding layer 45, the n-type Al0.14Ga0.86N/GaN MD-SLS cladding layers 40, the p-type GaN cladding layer 60, and the p-type Al0.14Ga0.86N/GaN MD-SLS cladding layer 55 because the refractive index of the of the active layer is larger than that of the n-type GaN cladding layer 45 and the p-type GaN cladding layer 60, the n-type Al0.14Ga0.86N/GaN M D-SLS layer 40, and the p-type Al0.14Ga0.86N/GaN MD-SLS cladding layer 60. Therefore, fundamental transverse mode operation is obtained.
However, for the structure shown in FIG. 1, it is difficult to reduce the defect density to the order of less than 108 cmxe2x88x922, because the lattice constants of AlGaN, InGaN, and GaN differ sufficiently different from each other that defects are generated in the structure as a way to release the strain energy whenever the total thickness of the n-type In0.1Ga0.9N layer 35, the In0.02Ga0.98N/In0.15Ga0.85N MQW active layer 50, the n-type AL0.14Ga0.86N/GaN MD-SLS cladding layer 40, the p-type Al0.14Ga0.86N/GaN MD-SLS cladding layer 65, and the p-type Al0.2Ga0.8N cladding layer 55 exceeds the critical thickness. The defects result from phase separation and act as absorption centers for the lasing light, causing decreased light emission efficiency and increased threshold current. The result is that the operating current becomes large, which in turn causes reliability to suffer.
Moreover, the ternary alloy system of InGaN is used as an active layer in the structure shown in FIG. 1. In this case, the band gap energy changes from 1.9 eV for InN to 3.5 eV for GaN. Therefore, ultraviolet light which has an energy level higher than 3.5 eV cannot be obtained by using an InGaN active layer. This presents difficulties, since ultraviolet light is attractive as a light source for the optical pick up device in, for example, higher density optical disc memory systems and other devices.
To better understand the defects which result from phase separation in conventional ternary materials systems, the mismatch of lattice constants between InN, GaN, and AlN must be understood. The lattice mismatch between InN and GaN, between InN and AlN, and between GaN and AlN, are 11.3%, 13.9%, and 2.3%, respectively. Therefore, an internal strain energy accumulates in an InGaAlN layer, even if the equivalent lattice constant is the same as that of the substrate due to the fact that equivalent bond lengths are different from each other between InN, GaN, and AlN. In order to reduce the internal strain energy, there is a compositional range which phase separates in the InGaAlN lattice mismatched material system, where In atoms, Ga atoms, and Al atoms are inhomogeneously distributed in the layer. The result of phase separation is that In atoms, Ga atoms, and Al atoms in the InGaAlN layers are not distributed uniformly according to the atomic mole fraction in each constituent layer. In turn, this means the band gap energy distribution of any layer which includes phase separation also becomes inhomogeneous. The band gap region of the phase separated portion acts disproportionately as an optical absorption center or causes optical scattering for the waveguided light. As noted above, a typical prior art solution to these problems has been to increase drive current, thus reducing the life of the semiconductor device.
As a result, there has been a long felt need for a semiconductor structure which minimizes lattice defects due to phase separation and can be used, for example, as a laser diode which emits blue or UV light at high efficiency, and for other semiconductor structures such as transistors.
The present invention substantially overcomes the limitations of the prior art by providing a semiconductor structure which substantially reduces defect densities by materially reducing phase separation between the layers of the structure. This in turn permits substantially improved emission efficiency.
To reduce phase separation, it has been found possible to provide a semiconductor device with InGaAlN layers having homogeneous In content, Al content, Ga content distribution in each layer. In a light emitting device, this permits optical absorption loss and waveguide scattering loss to be reduced, resulting in a high efficiency light emitting device.
A quaternary material system such as InGaAlN has been found to provide, reproducibly, sufficient homogeneity to substantially reduce phase separation where the GaN mole fraction, x, and the AlN mole fraction, y, of all the constituent layers in the semiconductor structure satisfy the condition that x+1.2y nearly equals a constant value.
A device according to the present invention typically includes a first layer of InGaAlN material of a first conductivity, an InGaAlN active layer, and a layer of InGaAlN material of an opposite conductivity successively formed on one another. By maintaining the mole fractions essentially in accordance with the formula x+1.2y equals a constant, for example on the order of or nearly equal to one, the lattice constants of the constituent layers remain substantially equal to each other, leading to decreased generation of defects.
In an alternative embodiment, the semiconductor structure is fabricated essentially as above, using a quaternary materials system to eliminate phase separation and promote homogeneity across the layer boundaries. Thus, as before, the first cladding layer is a first conduction type and composition of InGaAlN, the active layer is InGaAlN of a second composition, and the second cladding layer is an opposite conduction type of InGaAlN having the composition of the first layer. However, in addition, the second cladding layer has a ridge structure. As before, the optical absorption loss and waveguide scattering loss is reduced, leading to higher efficiencies, with added benefit that the optical field is able to be confined in the lateral direction in the active layer under the ridge structure. This structure also permits fundamental transverse mode operation.
In a third embodiment of the invention, suited particularly to implementation as a laser diode, the semiconductor structure comprises a first cladding of a first conduction type of an In1xe2x88x92x1xe2x88x92y1Gax1Aly1N material, an active layer of an In1xe2x88x92x2xe2x88x92y2Gax2Aly2N material, and a second cladding layer of an opposite conduction type of an In1xe2x88x92x3xe2x88x92y3Gax3Aly3N material, each successively formed on the prior layer. In such a materials system, x1, x2, and x3 define the GaN mole fraction and y1, y2, and y3 define the AlN mole fraction and x1, y1, x2, y2, x3, and y3 have a relationship of 0 less than x1+y1 less than 1, 0 less than x2+y2 less than 1,0 less than x3+y3 less than 1, 1 less than =x1/0.80+y1/0.89, 1 less than =x2/0.80+y2/0.89, 1 less than =x3/0.80+y3/0.89, EgInN(1xe2x88x92x1xe2x88x92y1)+EgGaNx1+EgAlNy1 greater than EgInN(1xe2x88x92x2xe2x88x92y2)+EgGaNx2+EgAlNy2, and EgInN(1xe2x88x92x3xe2x88x92y3)+EgGaNx3+EgAlNy3 greater than EgInN(1xe2x88x92x2xe2x88x92y2)+EgGaNx2+EgAlNy2, where EgInN, EgGaN, and EgAlN are the band gap energy of InN, GaN, and AlN, respectively.
To provide a reproducible semiconductor structure according to the above materials system, an exemplary embodiment of InGaAlN layers have Ga content, x, and Al content, y, which satisfy the relationship 0 less than x+y less than 1 and 1 less than =x/0.80+y/0.89. As before, this materials system permits reduction of the optical absorption loss and the waveguide scattering loss, resulting in a high efficiency light emitting device. Moreover, the band gap energy of the InGaAlN of an active layer becomes smaller than that of the first cladding layer and the second cladding layer when x1, y1, x2, y2, x3, and y3 have a relationship of 0 less than x1+y1 less than 1, 0 less than x2+y2 less than 1, 0 less than x3+y3 less than 1, 1 less than =x1/0.80+y1/0.89, 1 less than =x2/0.80+y2/0.89, 1 less than =x3/0.80+y3/0.89, EgInN(1xe2x88x92x1xe2x88x92y1)+EgGaNx1+EgAlNy1 greater than EgInN(1xe2x88x92x2xe2x88x92y2)+EgGaNx2+EgAlNy2, and EgInN(1xe2x88x92x3xe2x88x92y3)+EgGaNx3+EgAlNy3 greater than EgInN(1xe2x88x92x2xe2x88x92y2)+EgGaNx2+EgAlNy2. Under these conditions, the injected carriers are confined in the active layer. In at least some embodiments, it is preferable that the third light emitting device has an InGaAlN single or multiple quantum well active layer whose GaN mole fraction, xw, and AlN mole fraction, yw, of all the constituent layers satisfy the relationship of 0 less than xw+yw less than 1 and 1 less than =x/0.80+y/0.89.
One of the benefits of the foregoing structure is to reduce the threshold current density of a laser diode. This can be achieved by use of a single or multiple quantum well structure, which reduces the density of the states of the active layer. This causes the carrier density necessary for population inversion to become smaller, leading to a reduced or low threshold current density laser diode.
It is preferred that in the third light emitting device, the condition of xs+1.2ys nearly equals to a constant valuexe2x80x94on the order of or near onexe2x80x94is satisfied, wherein xs and ys are the GaN mole fraction and the AlN mole fraction, respectively in each the constituent layers. AS before, this causes the lattice constants of the each constituent layers to be nearly equal to each other, which in turn substantially minimizes defects due to phase separation
In a fourth embodiment of the present invention, the semiconductor structure may comprise a first cladding layer of a first conduction type of a material In1xe2x88x92x1xe2x88x92y1Gax1Aly1N, an In1xe2x88x92x2xe2x88x92y2Gax2Aly2N active layer, and a second cladding layer of an opposite conduction type of a material In1xe2x88x92x3xe2x88x92y3Gax3Aly3N, each successively formed one upon the prior layer. In addition, the second cladding layer has a ridge structure. For the foregoing materials system, x1, x2, and x3 define the GaN mole fraction, y1, y2, and y3 define the AlN mole fraction, and x1, y1, x2, y2, x3, and y3 have a relationship of 0 less than x1+yl less than 1,0 less than x2+y2 less than 1,0 less than x3+y3 less than 1, 1 less than =x1/0.80+y1/0.89, 1 less than =x2/0.80+y2/0.89, 1 less than =x3/0.80+y3/0.89, EgInN(1xe2x88x92x1xe2x88x92yl)+EgGaNx1+EgAlNy1 greater than EgInN(1xe2x88x92x2xe2x88x92y2)+EgGaNx2+EgAlNy2, and EgInN(1xe2x88x92x3xe2x88x92y3)+EgGaNx3+EgAlNy3 greater than EgInN(1xe2x88x92x2xe2x88x92y2)+EgGaNx2+EgAlNy2, where EgInN, EgGaN, and EgAlN are the bandgap energy of InN, GaN, and AlN, respectively.
As with the prior embodiments, each of the InGaAlN layers have a homogeneous In content, Al content, and Ga content distribution, which can be obtained reproducibly when Ga content, x, Al content, y, of each InGaAlN layer satisfies the relationship 0 less than x+y less than 1 and 1 less than =x/0.80+y/0.89. The band gap energy of the InGaAlN active layer becomes smaller than that of the first cladding layer and the second cladding layer when x1, y1, x2, y2, x3, and y3 have a relationship of 0 less than x1+y1 less than 1,0 less than x2+y2 less than 1,0 less than x3+y3 less than 1, 1 less than =x1/0.80+y1/0.89, 1 less than =x2/0.80+y2/0.89, 1 less than =x3/0.80+y3/0.89, EgInN(1xe2x88x92x1xe2x88x92y1)+EgGaNx1+EgAlNy1 greater than EgInN(1xe2x88x92x2xe2x88x92y2)+EgGaNx2+EgAlNy2, and EgInN(1xe2x88x92x3xe2x88x92y3)+EgGaNx3+EgAlNy3 greater than EgInN(1xe2x88x92x2xe2x88x92y2)+EgGaNx2+EgAlNy2. Similar to the prior embodiments, the injected carriers are confined in the active layer and the optical field is confined in the lateral direction in the active layer under the ridge structure, producing a fundamental transverse mode operation.
Also similar to the prior embodiments, the fourth embodiment typically includes an InGaAlN single or multiple quantum well active layer whose GaN mole fraction, xw, and AlN mole fraction, yw of all the constituent layers satisfy the relationship of 0 less than xw+yw less than 1 and 1 less than =x/0.80+y/0.89. Also, the condition xs+1.2ys nearly equals to a constant value on the order of or near one is typically satisfied, where xs and ys are the GaN mole fraction and the AlN mole fraction, respectively in each constituent layer. Similar parameters apply for other substrates, such as sapphire, silicon carbide, and so on.
The foregoing results may be achieved with conventional processing temperatures and times, typically in the range of 500xc2x0 C. to 1000xc2x0 C. See xe2x80x9cGrowth of high optical and electrical quality GaN layers using low-pressure metalorganic chemical vapor deposition,xe2x80x9d Appl. Phys. Lett. 58 (5), Feb. 4, 1991 p. 526 et seq.